Fundamental Concepts of Organic Reaction Mechanism

As we know the branch of chemistry that deals with the study of hydrocarbons and their derivatives is known as Organic Chemistry. But are you guys aware regarding the concepts of organic reaction mechanism? In this chapter, let us now understand the fundamental concepts of organic reaction mechanism in detail.

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Introduction to Organic Chemistry

The Shapes of Carbon Compounds

Both ‘s’ and ‘p’ orbitals are involved in hybridization, inorganic or carbon compounds which further leads to three types of hybridization which are sp³ (in alkanes) – Tetrahedral in shape sp² (in alkenes) – Linear molecule – Planar structure sp (in alkynes).

Functional Group

The functional groups are atom or group of atoms which are further joined in a specific manner and determines the chemical properties of the organic compound. Few of the examples are the hydroxyl group (—OH), carboxylic acid group (—COOH) and aldehyde group (—CHO) etc.

Homologous Series

 A homologous series refers to as a family of organic compounds that have similar chemical properties, the same functional group, and the successive members differ from each other in the molecular formula by —CH2 units. By the same general molecular formula, the members of a homologous series can be represented.

Nomenclature of Organic Compounds

Common name: Organic compounds were named after the sources of origin, before the IUPAC system of nomenclature. For example, urea was so named because it was obtained from the urine of mammals. Formic acid was so named since it was extracted from red ants called Formica.

The Fundamental Concepts in Organic Reaction Mechanism

Fission of a Covalent Bond

A covalent bond can undergo Fission in two ways:

• Homolytic Fission: Also referred to as Homolysis, Homolytic fission refers to the process wherein each of the atoms acquires one of the bonding electrons.
• Heterolytic Fission: Also referred to as Heterolysis, Heterolytic Fission refers to the process wherein when the bond is broken, one of the atoms acquires both of the bonding electrons.

In case B is more electronegative than A, further to acquires both the bonding electrons and becomes negatively charged. The products of heterolytic fission are ions.

Organic Reaction Mechanism: Inductive Effect

The frequently observed electron displacement effects in the substrate molecules are as following: it is a permanent effect which comes into existence when an electron is withdrawing or an electron donating group is attached to a chain of singly bonded carbon atoms.

The displacement of sigma-electrons along a saturated carbon chain due to the presence of an electron withdrawing group or electron repelling group at one end of the chain resulting in the development of partial positive or partial negative charges in the decreasing order of magnitude is called an inductive effect or I effect.

Organic Reaction Mechanism: Electromeric Effect

Electromeric effect or E effect refers to the complete transfer of the shared pair of pie electrons of multiple bonds to one of the shared atoms in the presence of an attacking reagent.

Resonance Effect (Mesomeric Effect)

Resonance refers to the phenomenon in which a molecule is represented by several electronic structures which do not differ much in their energy contents and are obtained by the oscillation of pie electrons. Such structures are called canonical forms and the molecule is said to be a resonance hybrid of these canonical forms.

The permanent effect involving the transfer of electron relayed through pie electrons of multiple bonds in a chain of carbon atoms in a molecule is called the mesomeric effect. The mesomeric effect is a permanent effect and comes into existence in the following two cases:

• when electron withdrawing or electron pumping group is in conjugation with a pie bond.
• when an atom or group having at least one lone pair of electron is in conjugation with a pie bond.

Solved Examples for You

Question 1. Which of the following represents the correct IUPAC name for the compounds concerned?

1. 2, 2-Dimethylpentane or 2-Dimethylpentane
2. 2, 4, 7-Trimethyloctane or 2, 5, 7- Trimethyloctane
3. 2-Chloro-4-methylpentane or 4-Chloro-2-methylpentane
4. But-3-yn- l-ol or But-4-ol-yne.

Answer:

1. 2, 2-Demethylpentane
2. 2, 4, 7-Trimethyloctane.
3. Alphabetical order of substituents: 2- Chloro-4-methylpentane
4. But-3-yn-l-ol. Lower locant for the principal functional group, i.e., alcohol.

Question 2. Draw formulas for the first five members of each homologous series beginning with the following compounds,

1. H—COOH
2. CH₃COCH₃
3. H—CH=CH₂

Answer:

(a) CH₃—COOH
CH₃CH₂—COOH CH₃CH₂CH₂—COOH
CH₃CH₂CH₂CH₂—COOH

(b) CH₃COCH₃
CH₃COCH₂CH₃
CH₃COCH₂CH₂CH₃
CH₃COCH₂CH₂CH₂CH₃
CH₃CO(CH₃)₄CH₃

(c) H—CH=CH₂
CH₃CH=CH₂
CH₃CH₂CH=CH₂
CH₃CH₂CH₂CH=CH₂
CH₃CH₂CH₂CH₂CH=CH₂

Question 3. Which of the two: O2NCH2CH2O– or CH3CH2O– is more stable and why?

Answer: O₂N——<——- CH₂——<——- CH₂ —<——- O– is more stable than CH₃——<——-CH₂——<——-O- because NO₂ group has -I-effect and hence it tends to disperse the -ve charge on the O-atom. In contrast, CH₃CH₂ has +I-effect. It, therefore, tends to intensify the -ve charge and hence destabilizes it.

Question 4. Explain why (CH3)3C⁺ is more stable than CH₃C⁺H₂.
Answer: (CH₃)₃C⁺ has nine alpha hydrogens and has nine hyperconjugation structures while CH₃C⁺H₂ has three alpha hydrogens and has three hyperconjugation structures, therefore(CH₃)₃C⁺ is more stable than CH₃C⁺H₂.